An air eliminator is a medical device which is placed in the fluid line between a source of physiological fluid such as blood or I.V. fluid and a patient receiving the fluid. Gases which manifest themselves as micro-bubbles may be introduced into the fluid during administration, e.g., when changing administration sets or may appear due to a change in the temperature and/or pressure of the fluid being administered. For example, sometimes blood is delivered to a patient using pressure infusors; in that event blood is driven into the drip chambers of the I.V. set causing air bubbles which could find their way into the patient and cause an embolism. An air eliminator or air trap is intended to avoid that problem.
Conventional air eliminators usually include an elongated chamber having upper and lower ends. Physiological fluid from an elevated administration set is introduced into the upper end of the chamber and flows under gravity to the lower end of the chamber and thence to the patient. As the liquid flows through the chamber, any air or other gas entrained in the liquid tends to separate from the liquid and form bubbles which rise to the top of the chamber. A vent is provided at the top of the chamber which allows gas but not liquid to leave the chamber. Examples of such known gas eliminators are disclosed in U.S. Pat. Nos. 3,677,248 and 4,900,308.
While such known devices operate satisfactorily in many respects, they do have certain drawbacks. For example, the device in the former patent is supposed to eliminate air from an irrigation liquid used to clear away tissue from instruments being used by a surgeon. Apparently, that air may reduce the flushing efficiency. Thus, the liquid flowing through that air eliminator is not really being infused into the patient's circulatory system. As such, that device is not intended to trap all of the air in the liquid being administered. Accordingly, the liquid is introduced vertically into the top of the chamber and the flow is intended to drive air bubbles downward away from the vent opening at the top of the chamber. Resultantly, air bubbles are carried along in the fluid outflow from the chamber. Moreover, that device does not include a filter so that it would be inappropriate for use when administering a physiological fluid such as blood to a patient.
In the air eliminator described in the latter above patent, the physiological fluid is introduced laterally or diametrically into the top of the chamber. As a result, the incoming liquid carries to and impacts the opposite wall of the chamber and rolls upward as well as downward in the chamber causing turbulence and air bubbles in the downward flow. In the case of blood, the turbulence may also damage the blood cells.
In addition, that latter air eliminator is not particularly efficient in removing air from physiological fluids, particularly when the fluid is being administered at a relatively high pressure, e.g., above 300 mm Hg. See e.g., the article entitled Performance of Level 1 Air Eliminator at High and Low Flow Rates, by M. W. B. Hartmannsgruber, and N. Gravenstein, Annesthesiology Vol. 77, No. 3A, September 1992. It appears that because of the relatively small diameter of the chamber and the high fluid flow rate, air bubbles are pulled downward away from the vent opening at the top of the chamber. This problem is exacerbated due to the presence of a filter near the bottom of the chamber which further reduces the chamber flow crossection to an annular path only 0.1 in. wide thereby increasing the velocity of fluid even more. This produces a sucking effect which draws the air bubbles into the fluid outflow from the chamber.
Additionally, the air eliminator in the latter patent has to be oriented vertically in order to operate properly. Therefore, a separate bracket must be provided to secure the air eliminator to a support to maintain the device in a vertical orientation. This adds to the cost of the device and makes it more difficult to place the device near the patient.